The present invention relates generally to the field of materials having a low index of refraction. In particular, the present invention relates to materials suitable for use in the field of optoelectronics.
The index of refraction (“n”) is a fundamental property of optical materials. It determines the speed of light in a material (“v”) given by the relationv=c/nwhere “c” is the speed of light in a vacuum. The index of refraction results from distortion of the electronic cloud of atoms or molecules in an electric field at optical frequencies. Its value is determined by the density of electrons per unit volume, also known as the volume polarizability. The index of refraction can change with wavelength of light, which is referred to as the dispersion of the refractive index. Additionally, the index of refraction can vary within a material. For instance, the index of refraction can be different along different directions of a material, referred to as birefringence. The index of refraction can also change with the intensity of optical radiation, resulting in many optical effects such as optical Kerr effect, four wave mixing and frequency doubling, as well as many other nonlinear optical effects.
The index of refraction of a material has both a real, i.e. non-absorbing, and an imaginary, i.e. absorbing, component. The relative contribution of each component to the index of refraction of a material depends upon the degree of absorption of the material. As the incident optical radiation approaches the absorption band of the material it is passing through, it is possible to effect electronic transitions. This can result in absorption, luminescence, and non linear optical effects such as light amplification and two photon absorption. Many optical devices, such as waveguides, lenses, gratings, electro-optical modulators and frequency doublers, and the like, utilize the real component of the refractive index.
Many devices utilize the spatial variation of refractive index to control and manipulate light to perform useful functions. For example, an optical waveguide transports light along a predefined path which consists of a guiding region or core surrounded by a cladding which has a lower index of refraction than the core. The waveguide properties are determined by the difference in the refractive indices of the core and cladding, as well as the dimensions of the waveguide. Typical differences in the indices of refraction (“Δn”) are from 0.001 to 0.01.
A conventional waveguide cannot be bent more than a limited number of degrees as light will radiate from it rather than travel around such bends. As a result, current photonic devices have to be of dimensions that accommodate varying “S-shaped” bends. Such designs are the result of the relatively small Δn that is available in conventional optical waveguides.
With the increased use of light to transmit data, there is a desire to use waveguides on printed wiring boards. However, the limited ability to bend such waveguides means that only straight waveguides are used or else much of the printed wiring board area is lost to the large bends required by conventional waveguide materials. The trend to increasing the density of components of printed wiring boards is inconsistent with the required large waveguide bends of conventional waveguides. To increase the density of optical integrated circuits on printed wiring boards and in photonic component applications, there is a need for optical waveguides that can support such small bend radii.
Another device for changing the path of light is a Bragg grating, which is a grating consisting of periodic regions of high and low refractive indices. Such gratings transmit or reflect a narrow band of radiation and can act as mirrors or narrow band filters. When the ratio of the indices of the high to low index regions exceeds a certain value it is possible to form photonic band gap structures which reflect radiation at all angles and polarizations.
Bragg gratings have many uses for optical communication systems, such as DWDM, pulse shaping, add-drop switches and the like. The index difference obtained in typical optical fiber Bragg gratings is about 0.001, obtained by UV radiation of photosensitized glass. Thin film filters for DWDM applications use inorganic materials with larger index differences, for example SiO2 (n=1.46) and TiO2 (n=2.7), which are applied by chemical vapor deposition. These gratings can also be used as antireflection coatings for lenses or photoresists used in high resolution lithography. A problem with Bragg gratings is that they can cause distortion of pulses due to the periodic high and low refractive index structure and due to the dispersion of the index of refraction of the materials. There is, therefore, a need for Bragg gratings that are fabricated from lower dispersion materials.
It is possible to fold periodic layers into a cylinder to form a light pipe with a periodically varying index wall. The guide, or center of the pipe, itself can be filled with air or a liquid or solid material. An air filled guide surrounded by an all dielectric mirror is advantageous as an optical pulse is not distorted due to dispersion of refractive index, as occurs in a normal glass waveguide. It is also possible to bend the light path with very small bend radii. However, it is difficult to obtain compatible materials having very low indices of refraction (“nL”) and very high indices of refraction (“nH”), in order to obtain the required large ratios of nH/nL. There is a need for ultralow and very high refractive index materials to realize photonic band gap devices.
Light can also be bent by way of gray scale variations of refractive index. Such gray scale variation of index allows light to be bent in unusual ways, for example in diffractive optical elements or rugate filters. Switchable gratings may also be produced from such gray scale variations. For example a polymer dispersed liquid crystal (“PDLC”) medium consists of liquid crystal microdroplets which orient in an electric field and effect a change in refractive index. Such a holographic PDLC medium can be used as tunable mirror, an add-drop multiplexer, and the like.
Certain porous materials are known for optical applications. For example, Rossi et al., High-quality Porous-Silicon Buried Waveguides, Applied Physics Letters, vol. 78, no. 20, pp 3003-3005, May 14, 2001, disclose waveguides having both a porous core and cladding material. The porous nature of such silicon waveguide material is formed by applying a certain current density for a set period of time. The pore sizes and pore distribution in such materials is not easily controlled using such process. It would be desirable to prepare such porous materials where the pore size and pore distribution can be precisely controlled. In this way, it would be possible to control or tailor the index of refraction of the resulting porous optical material.
It is clear from the above that there is a need for new optical materials to enable new devices and to improve the performance of current materials.